Gas Turbines with Multiple Gas Flow Paths

ABSTRACT

A representative gas turbine includes: a high pressure spool; a high pressure compressor; a high pressure turbine mechanically coupled to the high pressure spool; a lower pressure spool; a lower pressure turbine mechanically coupled to the lower pressure spool; a fan having a first set of fan blades and a second set of fan blades, the second set of fan blades being located downstream of the first set of fan blades and being operative to rotate at a rotational speed corresponding to a rotational speed of the lower pressure spool; and a gear assembly mechanically coupled to the lower pressure spool and engaging the first set of fan blades such that rotation of the lower pressure spool rotates the first set of fan blades at a lower rotational speed than the rotational speed of the second set of fan blades.

BACKGROUND

1. Technical Field

The present disclosure generally relates to gas turbines.

2. Description of the Related Art

A typical single-spool gas turbine incorporates an intake that providesa flow of gas, e.g., air, to a compressor, a combustion section and aturbine in sequence. The turbine and the compressor are linked to eachother via a spool. In operation, energy generated by combustion of afuel-air mixture in the combustion section is converted to rotationalenergy by the turbine. The rotational energy is imparted to thecompressor via the spool for compressing additional gas received throughthe intake.

Multi-spool gas turbines also are known, each of which typicallyincludes multiple turbines and multiple compressors. Conventionally, thespools of a multi-spool gas turbine are concentric, with each beinglinked to a corresponding compressor-turbine pair.

SUMMARY

An exemplary embodiment of a gas turbine comprises: a high pressurespool; a high pressure compressor; a high pressure turbine mechanicallycoupled to the high pressure spool; a lower pressure spool; a lowerpressure turbine mechanically coupled to the lower pressure spool; a fanhaving a first set of fan blades and a second set of fan blades, thesecond set of fan blades being located downstream of the first set offan blades and being operative to rotate at a rotational speedcorresponding to a rotational speed of the lower pressure spool; and agear assembly mechanically coupled to the lower pressure spool andengaging the first set of fan blades such that rotation of the lowerpressure spool rotates the first set of fan blades at a lower rotationalspeed than the rotational speed of the second set of fan blades.

Another exemplary embodiment of a gas turbine comprises: a spool; aturbine mechanically coupled to the spool; a multi-stage fan having afirst set of blades and a second set of blades, the second set of bladesbeing located downstream of the first set of blades and being operativeto rotate with the spool; and means for enabling the first set of bladesto rotate at a lower rotational speed than the second set of blades.

Another exemplary embodiment of a multi-stage fan assembly for a gasturbine comprises: an epicyclic gear having a ring gear; and a first setof fan blades operative to rotate at a speed of the ring gear.

Other systems, methods, features and/or advantages of this disclosurewill be or may become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features and/oradvantages be included within this description and be within the scopeof the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale. Moreover, in the drawings, like reference numeralsdesignate corresponding parts throughout the several views.

FIG. 1 is a schematic diagram depicting an embodiment of a gas turbine.

FIG. 2 is a schematic diagram depicting another embodiment of a gasturbine

FIG. 3 is schematic diagram depicting another embodiment of a gasturbine.

FIG. 4 is a schematic diagram depicting another embodiment of a gasturbine

FIG. 5 is schematic diagram depicting another embodiment of a gasturbine.

FIG. 6 is schematic diagram depicting still another embodiment of a gasturbine.

FIG. 7 is schematic diagram depicting yet another embodiment of a gasturbine.

DETAILED DESCRIPTION

A representative embodiment of a gas turbine is shown schematically inFIG. 1. As shown in FIG. 1, gas turbine 100 includes a compressor 102, acombustion section 104 and a turbine 106. Thus, the embodiment of FIG.1, the compressor, combustion section and turbine define a gas flow paththat has a normal flow direction from the compressor to the turbine.

Notably, compressor 102 incorporates a first set of blades 108 and othersets of blades 110. The first set to blades 1U engages a gear assembly112. In this embodiment, the gear assembly is configured such that thefirst set of blades 108 rotates at a slower rotational speed than thatof the other sets of compressor blades 110. Notably, all of the bladesof the compressor and of the turbine are driven by a single spool 114.

With respect to the gear assembly 112, the embodiment of FIG. 1incorporates an epicyclic gear. Although capable of various dimensionsand arrangements, in this embodiment, the first set of blades 108 of thecompressor is coupled to the ring gear 116 of the epicyclic gear and thespool 114 is coupled to the sun gear 118. Note also that, in thisembodiment, the rotational axes of the planet gears 120 are fixed inposition relative to a non-moving portion of the gas turbine. By way ofexample, in some embodiments, the carrier that mounts the planet gearsis mounted to the turbine casing.

FIG. 2 is a schematic diagram depicting another embodiment of a gasturbine. As shown in FIG. 2, gas turbine 200 incorporates a compressor202, a combustion section 204 and a turbine 206. In this embodiment, thecompressor and turbine are coupled to a single spool 208 that engages agear assembly 210. Specifically, the gear assembly enables a first setof compressor blades 212 to rotate at a different speed than the speedat which a second set of compressor blades 214 rotate. Notably,compressor blades 214 include one or more set of blades that are locateddownstream of the first set of blades 212.

In this embodiment, gear assembly 210 incorporates an epicyclic gearthat engages the first set of blades via ring gear 216 and engages thespool 208 via sun gear 218. Note also that the planet gears 220 in thisembodiment are held by a carrier 222 that is fixed in position by beingmounted to turbine casing 224.

Gas turbine 200 also incorporates two gas flow paths. In particular, theturbine, the combustion section and a portion of the compressor arepositioned along a first gas flow path 230, which is radially locatedclosest to the gas turbine centerline 232. Radially outboard of thefirst gas flow path is a second gas flow path 234. Notably, the firstset of blades 212 of the compressor is configured to compress gastravelling along both the first and second gas flow paths. Specifically,each blade of the first set incorporates an inboard portion 236positioned along the first gas flow path and outboard portion 238positioned along the second gas flow path. An intermediate shroud 240 orother barrier is located between the two portions 236, 238. Thus, thecompressor (e.g., the first set of blades 212) mechanically divides theinlet flow of gas into separate inner and outer annular gas flow paths(230, 234). Therefore, only the gas travelling along the inner gas flowpath 230 experiences additional stages of the compressor, the combustionsection and the turbine.

FIG. 3 is a schematic diagram of another embodiment of the gas turbine.As shown in FIG. 3, gas turbine 300 includes a compressor 302, acombustion section 304 and a turbine 306. Although depicted in proximityto the combustion section, various components such as compressors and/orturbines can be interposed between the compressor 302 and combustionsection and/or between combustion section and turbine 306. This is alsothe case for other embodiments.

Gas turbine 300 also incorporates a multi-stage fan 308 that is drivenby a spool 310. Specifically, a first set of fan blades 312 is driven bythe spool via a gear assembly 314 and a second set of blades 316 isdriven directly by the spool without any intervening gearing. In thisembodiment, the gear assembly comprises an epicyclic gear, of which thering gear 318 is coupled to the fan blades 312 and the sun gear 320 iscoupled to spool 310 for driving the fan blades 316. In operation,rotation of the spool causes the fan blades 316 to rotate at acorresponding speed, while the gear assembly 314 causes the fan blades312 to rotate at a different speed, which is typically slower. Note alsothat, in this embodiment, the axes of rotation of the planet gears 322are held by a carrier 324 that is fixed in position relative to anon-moving portion of the gas turbine such as a case or other stationarysupport.

Another embodiment of gas turbine is depicted schematically in FIG. 4.As shown in FIG. 4, gas turbine 400 incorporates a high pressurecompressor 402, a combustion section 404 and a high pressure turbine406. The high pressure compressor and high pressure turbine areinterconnected with and driven by a high spool 410. Spool 410 alsodrives a first set of blades 414 of the high pressure compressor via aninterposed gear assembly 412.

Gas turbine 400 also incorporates a lower pressure turbine 420 (e.g., alow pressure turbine) that is interconnected with and drives a low spool422. Spool 422 drives at least some of the blades of a multi-stage fan424. In particular, the multi-stage fan incorporates a first set ofblades 426, which is driven by spool 422 via an intervening gearassembly 428, and a second set of blades 430, which is driven at a speeddirectly corresponding to the rotational speed of the spool 422. In thisembodiment, the gear assembly incorporates an epicyclic gear, the ringgear 432 of which is coupled to the blades 426 and the sun gear 434 ofwhich is coupled to spool 422. Note also that, in this embodiment, theaxes of rotation of the planet gears 436 are held by a carrier 438 thatis fixed in position relative to a non-moving portion of the gasturbine.

In operation, intake gas is acted upon by the first set of blades 426 ofthe multi-stage fan and then by the second set of blades 430, whichrotates at a faster speed than the first set of blades 426. Thereafter,the gas is diverted to flow either along a first gas flow path 440 or asecond gas flow path 442. Along the first gas flow path, the gasinteracts with inboard portions 444 of a first set of compressor blades414 that rotate at a speed that is slower than a subsequent set ofblades of the compressor 402. This speed differential is facilitated bythe gear assembly 412.

Gear assembly 412 incorporates another epicyclic gear, the ring gear 450of which is coupled to the compressor blades 414 and the sun gear 452 ofwhich is coupled to spool 410. Note also that, in this embodiment, theaxes of rotation of the planet gears 454 are held by a carrier 456 thatis fixed in position relative to a non-moving portion of the gasturbine. Additionally, the sun gear 452 includes a central aperture 458through which spool 422 extends.

After passing through the compressor 402, gas flowing along the firstgas flow path travels through the combustion section 404, the highpressure turbine 406 and the lower pressure turbine 420 in sequence. Incontrast, downstream of the multi-stage fan 424, gas flowing along thesecond gas flow path is acted upon by outboard portions 446 of the firstset of compressor blades 414. Thereafter, the gas does not pass throughadditional stages of the compressor, the combustion section, the highpressure turbine or the lower pressure turbine.

FIG. 5 schematically depicts another embodiment of the gas turbine. Asshown in FIG. 5, gas turbine 500 incorporates various components thatwill be described generally in sequential order starting from theupstream or inlet side. In this regard, gas entering inlet 502encounters a strut assembly 504, an inboard portion of which isconfigured as a strut 506 that mounts an adjustable inlet guide vane508. The vane 508 is positioned between an outer shroud 510 and an innershroud 512, a portion of which divides the strut assembly into inboard(strut) and outboard (vane) portions. Notably, the inner shroud dividesthe flow of gas such that an outer annular gas flow path 520 is createdthat surrounds an inner annular gas flow path 540.

After passing the strut assembly, gas travelling along the gas flow path520 interacts with tip rotor blades 522 of an outboard, auxiliary fan524. In this embodiment, the tip rotor blades are mounted to distal endsof radially extending spokes 526 of a Cortland Burge Wheel (CBW) that isdriven by a spool 538. The spokes of this embodiment are not aerodynamicbodies, in that the spokes are designed not to impact flowcharacteristics of the gas travelling through the spokes substantially.

Downstream of the tip rotor blades 522 of the auxiliary fan, gastravelling along the gas flow path 520 encounters an adjustable exitguide vane 528. Thereafter, the gas is directed through the annular pathbetween adjacent shrouds 510, 512 before exiting as exhaust.

Along the gas flow path 540 downstream of the spokes 526 of the CBW, thegas encounters an adjustable inlet guide vane 542 that, in thisembodiment, incorporates a fixed leading edge portion 544 and a variabletrailing edge portion 546. Downstream of the vane 542, a multi-stage fan550 is positioned. In this embodiment, the multi-stage fan incorporatesa first set of blades 552 and a second set of blades 554 between whichis interposed a set of adjustable stator vanes 556. Notably, the secondset of fan blades 554 is driven by a spool 558 at a higher rotationalspeed than the rotational speed of the first set of blades 552. In thisregard, a gear assembly 560 is mechanically coupled between the firstset of fan blades and the spool 558 so that the blades of the first setexhibit a slower rotational speed.

In the embodiment of FIG. 5, the gear assembly 560 incorporates anepicyclic gear, the ring 562 of which is coupled to the first set of fanblades and the sun gear 564 of which is coupled to the spool 558 fordriving the second set of fan blades. Note that the planet gears 566 areheld by a carrier 568 that is fixed in position by mounting to anon-moving portion of the gas turbine. In this embodiment, the carrieris attached to the fixed leading edge portion 544 of the vane 542. Notealso that the sun gear 564 includes an aperture through which spool 538passes.

Downstream of the multi-stage fan, the gas flow diverges again. Thistime, the inner annular gas flow path 540 is separated into a gas flowpath 570 and a gas flow path 572. Each of these gas flow pathsincorporates an adjustable inlet guide vane (574, 576) at its respectiveinlet. In this embodiment, the vanes 574, 576 are separatelycontrollable to provide highly adjustable flow controllability.

A set of blades 578 of a high pressure compressor 580 is locateddownstream of the vanes 574, 576 along each of the gas flow paths 570,572. Specifically, in this embodiment, the set of blades 578 forms afirst stage of the high pressure compressor, with outboard portions 579being located along the path 572 and inboard portions 581 being locatedalong the path 570. Others of the sets of blades of the high pressurecompressor (in this case, all of the subsequent sets of blades) arelocated only along the innermost gas flow path 570. Despite being drivenby the same spool 582, the first set of blades is driven at a slowerrotational speed than at least some of the other blades of the highpressure compressor. This is accomplished by mechanically coupling agear assembly 584 between the first set of blades and the spool 582.

In this embodiment, the gear assembly includes an epicyclic gear, thering gear 586 of which is coupled to the first set of blades and the sungear 588 of which is coupled to the spool. Note that, in thisembodiment, carrier 590 that holds the planet gears 592 is fixed inposition relative to the gas turbine by attachment to the turbine casing594. Note also that the sun gear 588 includes an aperture through whichspools 558 and 538 pass.

Returning briefly to the radially central gas flow path 572, downstreamof the outboard portions of the first set of compressor blades, gasencounters an adjustable guide vane 596. Thereafter, the gas travels theremainder of the gas flow path 572 as defined by the turbine casing 594and the inner shroud 512. In contrast, after departing the high pressurecompressor, gas travelling along the gas flow path 570 is directedthrough a combustion section 598 and thereafter through an adjustableinlet guide vane 602 prior to entering a high pressure turbine 604. Notethat the high pressure turbine is used to drive spool 582.

An adjustable exit guide vane 606 is positioned downstream of the highpressure turbine. The vane 606 also functions as an adjustable inletguide vane for an intermediate pressure turbine 608. The intermediatepressure turbine drives spool 558, which powers the multi-stage fan 550.

An adjustable exit guide vane 610 is located downstream of theintermediate pressure turbine that also functions as an adjustable inletguide vane for a low pressure turbine 612. The low pressure turbinedrives the spool 558, which powers the auxiliary fan 524. An adjustableexit guide vane 614 is located downstream of the low pressure turbine.

In the embodiment of FIG. 5, a mixer 620 is used to mix gas exiting fromthe gas flow paths 570, 572. Additionally or alternatively, fan air fromthe gas flow path 520 can exit through a separate nozzle, which may ormay not be variable in exit area (not shown).

In operation, an embodiment of a gas turbine such as that depicted inFIG. 5 can potentially provide enhanced flow controllability andcorresponding improvements in performance at various power and operatingconditions. By way of example, allowing the low pressure turbine todrive the auxiliary fan can potentially enable improved matching of theflow and power requirements of the auxiliary fan with the flow and poweroutput of the low power turbine. In some embodiments, this matching canbe enhanced by the use of adjustable geometry inlet and exit guide vanesassociated with the auxiliary fan, as well as adjustable geometry inletand exit guide vanes of the low pressure turbine.

Additionally or alternately, adjustable geometry inlet and exit vanesaffecting the outboard portions of the blades of the high pressurecompressor can enable an improved matching of the flow and pressure ofthe gas flow path 570 with the exit flow and pressure of the gas flowpath 572. Such improved matching can potentially reduce turbine screech,for example.

Additionally, or alternatively, improved matching of the speed, flow andpower of each spool can be achieved by the adjustable geometry inlet andexit vanes of each of the turbines.

Additionally or alternatively, use of a gear assembly can enable slowerrotor tip speeds of at least some of the upstream blades of the highpressure compressor and higher rotor tip speeds of at least some of thedownstream blades of the high pressure compressor. Similarly, use of agear assembly can enable at least one upstream stage of a multi-stagefan to exhibit a slower rotor tip speed than the rotor tip speed atleast one downstream stage.

FIG. 6 is a schematic diagram of another embodiment of the gas turbine.As shown in FIG. 6, gas turbine 600 includes a compressor 602, acombustion section 604 and a turbine 606. Although depicted in proximityto the combustion section, various components such as compressors and/orturbines can be interposed between the compressor 602 and combustionsection and/or between combustion section and turbine 606. This is alsothe case for other embodiments.

Gas turbine 600 also incorporates a multi-stage fan 608 that is drivenby a single spool 610. Specifically, a first set of fan blades 612 isdriven by the spool via a gear assembly 614 and a second set of blades616 is driven directly by the spool without any intervening gearing. Inthis embodiment, the gear assembly comprises an epicyclic gear, of whichthe planet carrier 624 is coupled to the fan blades 612 and the sun gear620 is coupled to spool 610 for driving the fan blades 616. Inoperation, rotation of the spool causes the fan blades 616 to rotate ata corresponding speed, while the gear assembly 614 causes the fan blades612 to rotate at a different, typically slower, speed. Note also that,in this embodiment, the axes of rotation of the planet gears 622 areheld by a ring gear 618 that is fixed in position relative to anon-moving portion of the gas turbine.

Another embodiment of gas turbine is depicted schematically in FIG. 7.As shown in FIG. 7, gas turbine 700 incorporates a high pressurecompressor 702, a combustion section 704 and a high pressure turbine706. The high pressure compressor and high pressure turbine areinterconnected with and driven by a spool 710. Spool 710 also drives afirst set of blades 714 of the high pressure compressor via aninterposed gear assembly 712.

Gas turbine 700 also incorporates a lower pressure turbine 720 (e.g., alow pressure turbine) that is interconnected with and drives a spool722. Spool 722 drives at least some of the blades of a multi-stage fan724. In particular, the multi-stage fan incorporates a first set ofblades 726, which is driven by spool 722 via an intervening gearassembly 728, and a second set of blades 730, which is driven at a speeddirectly corresponding to the rotational speed of the spool 722. In thisembodiment, the gear assembly incorporates an epicyclic gear, the ringgear 732 of which is coupled to the blades 726 and the sun gear 734 ofwhich is coupled to spool 722. Note also that, in this embodiment, theaxes of rotation of the planet gears 736 are held by a carrier 738 thatis fixed in position relative to a non-moving portion of the gasturbine.

In operation, intake gas is acted upon by the first set of blades 726 ofthe multi-stage fan and then by the second set of blades 730, whichrotates at a different, e.g., faster, speed than the first set of blades726. Thereafter, the gas is diverted to flow either along a first gasflow path 740 or a second gas flow path 742. Along the first gas flowpath, the gas interacts with inboard portions 744 of a first set ofcompressor blades 714 that rotate at a speed that is typically slowerthan a subsequent set of blades of the compressor 702. This speeddifferential is facilitated by the gear assembly 712 which incorporatesanother epicyclic gear, the ring gear 750 of which is coupled to thecompressor blades 702 and the planet carrier 752 which is coupled tocompressor blades 714. Note also that, in this embodiment, the axes ofrotation of the planet carrier 752 and ring gear 750 are held by a sungear 754 that is fixed in position relative to a non-moving portion ofthe gas turbine. Additionally, the sun gear 754 includes a centralaperture 756 through which spool 722 extends.

After passing through the compressor 702, gas flowing along the firstgas flow path travels through the combustion section 704, the highpressure turbine 706 and the lower pressure turbine 720 in sequence. Incontrast, downstream of the multi-stage fan 724, gas flowing along thesecond gas flow path is acted upon by outboard portions 746 of the firstset of compressor blades 714. Thereafter, the gas does not pass throughadditional stages of the compressor, the combustion section, the highpressure turbine or the lower pressure turbine.

It should be emphasized that the above-described embodiments are merelypossible examples of implementations set forth for a clear understandingof the principles of this disclosure. Many variations and modificationsmay be made to the above-described embodiments without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and protected by the accompanying claims.

1. A gas turbine comprising: a high pressure spool; a high pressurecompressor; a high pressure turbine mechanically coupled to the highpressure spool; a lower pressure spool; a lower pressure turbinemechanically coupled to the lower pressure spool; a fan having a firstset of fan blades and a second set of fan blades, the second set of fanblades being located downstream of the first set of fan blades and beingoperative to rotate at a rotational speed corresponding to a rotationalspeed of the lower pressure spool; and a gear assembly mechanicallycoupled to the lower pressure spool and engaging the first set of fanblades such that rotation of the lower pressure spool rotates the firstset of fan blades at a lower rotational speed than the rotational speedof the second set of fan blades.
 2. The gas turbine of claim 1, whereinthe gear assembly comprises an epicyclic gear having a sun gear,multiple planet gears and a ring gear.
 3. The gas turbine of claim 2,wherein: the gas turbine further comprises a casing surrounding at leasta portion of the fan; and rotational axes of the multiple planet gearsare fixed in position relative to the casing.
 4. The gas turbine ofclaim 2, wherein the sun gear is coupled to the lower pressure turbineand the ring gear is coupled to the first set of blades of the fan. 5.The gas turbine of claim 1, wherein the second set of blades of the fanis coupled to the lower pressure spool without any intervening gearing.6. The gas turbine of claim 1, wherein: the high pressure turbine andthe lower pressure turbine define a first gas flow path; the gas turbinedefines a second gas flow path located radially outboard of the firstgas flow path; and the fan is operative to provide flows of gas to boththe first gas flow path and the second gas flow path.
 7. The gas turbineof claim 6, wherein: the high pressure compressor has a first set ofblades; at least one of the first set of blades of the high pressurecompressor is a split rotor having an inboard portion and an outboardportion, the inboard portion being located along the first gas flowpath.
 8. The gas turbine of claim 7, wherein the outboard portion of thesplit rotor is located along the second gas flow path.
 9. The gasturbine of claim 1, wherein: the lower pressure spool is a low pressurespool and the lower pressure turbine is a low pressure turbine; and thegas turbine further comprises: an intermediate pressure spool; and anintermediate pressure turbine mechanically coupled to the intermediatepressure spool.
 10. The gas turbine of claim 9, further comprising anadjustable fan inlet guide vane operative to adjust gas flow provided toan inlet side of the fan.
 11. The gas turbine of claim 10, wherein: thehigh pressure turbine, the intermediate pressure turbine and the lowpressure turbine define a first gas flow path; the gas turbine defines asecond gas flow path located radially outboard of the first gas flowpath; and wherein the adjustable fan inlet guide vane is operative toadjust gas flow along the first gas flow path and the second gas flowpath.
 12. The gas turbine of claim 9, further comprising an auxiliaryfan coupled to and operative to be driven by the low pressure spool. 13.The gas turbine of claim 12, further comprising a Cortland Burge Wheelhaving a set of spokes and a set of tip rotors, wherein the auxiliaryfan comprises the set of tip rotors.
 14. The gas turbine of claim 13,wherein: the fan is operative to provide flows of gas to both the firstgas flow path and the second gas flow path; the gas turbine defines athird gas flow path located radially outboard of the second gas flowpath; and the set of tip rotors is located along the third gas flowpath.
 15. The gas turbine of claim 1, wherein: the gear assemblycomprises an epicyclic gear having a sun gear, multiple planet gears anda ring gear, the sun gear having an aperture; the sun gear is coupled tothe high pressure turbine; and the lower pressure spool passes throughthe aperture of the sun gear.
 16. A gas turbine comprising: a spool; aturbine mechanically coupled to the spool; a multi-stage fan having afirst set of blades and a second set of blades, the second set of bladesbeing located downstream of the first set of blades and being operativeto rotate with the spool; and means for enabling the first set of bladesto rotate at a lower rotational speed than the second set of blades. 17.The gas turbine of claim 17, wherein the means for enabling the firstset of blades to rotate comprises an epicyclic gear having a sun gear,multiple planet gears and a ring gear.
 18. The gas turbine of claim 18,wherein the sun gear is coupled to the second set of blades and the ringgear is coupled to the first set of blades.
 19. A multi-stage fanassembly for a gas turbine comprising: an epicyclic gear having a ringgear; and a first set of fan blades operative to rotate at a speed ofthe ring gear.
 20. The multi-stage fan assembly of claim 19, wherein:the epicyclic gear has a sun gear; and the fan comprises a second set offan blades operative to rotate at a speed of the sun gear.